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Roughly sunrise

Ian Stewart should have used a different example of precise prediction than the time of sunrise, which cannot, as he claims, be predicted within fractions of a second (25 September, p 28).

While the sun’s physical direction in space with respect to any location on the Earth can be predicted precisely, albeit within limits subject to irregularities in the rotation rate of the Earth, its apparent position is affected by refraction in the Earth’s atmosphere by an amount dependent upon atmospheric pressure and temperature, and this effect is particularly pronounced near the horizon.